The present invention relates to a metallic tubular structure having an improved collapse strength and also to a method of producing the same.
The term "collapse strength" in this specification is used to mean a strength of a tubular structure against collapse by an external pressure applied to the tubular structure. The tubular structure to which the invention pertains includes various members generally having a tubular form, particularly pipes, tubes and casing used in oil wells.
The current shortage of petroleum and natural gas resources has increased a tendency for deepening of oil and gas wells, which in turn tends to involve inclusion of hydrogen sulfide in the produced petroleum and gases. The tubes used in such wells, therefore, are required to have superior collapse strength, as well as high corrosion resistance.
However, corrosion resistance and collapse strength are generally considered as being incompatible with each other. More specifically, although the collapse strength can be increased through an increase of the yield strength by improvement of the material, i.e. by adjustment of components and heat-treatment, the increase in the yield strength is nothing but an increase in the tensile strength which is inevitably accompanied by a degragation in the resistance to corrosion. Therefore, there is a practical limit to the increase of the collapse strength through adjustment of the material and, hence, the improvement in the material alone cannot constitute an effective measure for improving the collapse strength of the pipes used in oil or gas wells.
In order to obtain pipes for use in oil wells usable under such severe condition, it is necessary to improve the collapse strength independently of the corrosion resistance. To this end, various methods have been proposed as listed below.
(1) To effect a contraction processing on pipe
(2) To omit straightening steps
(3) To conduct the straightening step in a warm state
(4) To effect water cooling following quench-tempering.
The above-mentioned methods, however, have their own drawbacks or shortcomings.
For instance, the above-mentioned method (1) suffers from the following problem. Contraction processing is effected to increase only the circumferential yield strength, which directly contributes to the increase in the collapse strength, while maintaining the tensile strength unchanged. The problem arises from the use of a specific contracting means. Namely, the contracting means includes a plurality of circumferential segments. It is quite difficult to obtain uniform contact of the circumferential segments over the entire periphery of the steel pipe and, therefore, the rate of increase in the yield strength fluctuates over the circumference of the steel pipe. With this method, therefore, it is not possible to attain a stable and effective improvement in the collapse strength.
The method (2) mentioned above is based upon a finding that a reduction in the collapse strength is often caused by residual compression stress in the inner peripheral surface of the steel pipe caused by a straightening which is conducted as the final step of the pipe producing process. If this straightening step is to be omitted, it is necessary to carry out the preceding steps at an impractically high precision. In fact, it is quite difficult to produce steel pipes meeting the customer's precision requirements without the step of straightening, particularly when the pipe diameter is small.
The method (3) is intended for eliminating the generation of the aforementioned residual stress by conducting the straightening at an elevated temperature. This method does not involve any substantial problems but, as in the case of the method (2) mentioned before, the elimination of residual stress is not a positive measure and cannot provide sufficient effect by itself.
The method (4) has been proposed in Japanese Patent Laid-open No. 33424/1981. This method is based upon a technical idea that the collapse strength can be increased by imparting residual tensile stress of a level higher than 20 Kg/mm.sup.2 but lower than the yield stress to the inner peripheral surface, and teaches that such residual tensile stress is obtainable by a water cooling subsequent to the tempering. This prior art, however, does not make clear the relationship between the condition of water cooling and the level of the residual stress. The method (4), therefore, is not considered as being an established method which can stably improve the collapse strength of the steel pipe. It is to be pointed out also that the idea concerning the relationship between the collapse strength and the residual tensile stress is incorrect, as will be understood from the following brief explanation. To sum up, the above-mentioned technical idea necessitates an assumption or hypothesis that the collapse of a pipe under application of external force starts at the inner side of the pipe. Such an assumption does not always match the actual case. Namely, when a residual stress is previously developed in the circumferential direction of the steel pipe, the collapse does not always begin with the inner surface of the pipe but in some cases it begins with the external surface of the pipe when the residual circumferential stress in the inner peripheral surface of the pipe exceeds a certain level. The above-mentioned assumption can by no means applies to such a case. It would be not too much to say that the above-mentioned technical idea is an empty theory. Such an empty theory can by no means provide a stable effect.
Thus, all of the methods proposed hitherto for improving the collapse strength regardless of the corrosion resistance are imperfect and unsatisfactory.